Studio della lineshape del segnale nell'analisi per la ricerca del bosone di Higgs nel canale HZZ4l a CMS

The Standard Model of elementary particles is a relativistic quantum field theory that describes the particles observed in nature and their interactions, except for gravitation. The predictions of this theory coincide with obser- vations in experiments with an incredible precision, making it the most stringently tested existing scientific theory. The only missing piece in the spectrum of the theory is the Higgs boson, the quantum of the field believed to be responsible for the spontaneous breaking of the SM gauge symme- try that also represents the key to the origin of the mass of fundamental particles. The Compact Muon Solenoid (CMS) is one of the four main experiments at the Large hadron Collider (LHC) and the Higgs search is one of the more important parts of its physics program. In July 2012 the CMS and ATLAS experiment announced the discovery of a new boson at a mass around 125 GeV, with properties compatible with the SM Higgs boson. In this thesis, I present my work in the CMS experiment for the search of the standard model Higgs boson decaying into four leptons. In particular, I have focused my work on the study of the signal lineshape. My thesis is structured as follows. Chapter 1 introduces the theoretical framework of the Standard Model together with the Higgs boson properties. It further offers an overview of the constrains on the Higgs boson mass before the LHC era and nowadays, on an experimental and on a theoretical point of view. In Chapter 2 the LHC accelerator is introduced and the experimental setup of the CMS detector and the low-level reconstruction is summarized. Chapter 3 concerns the analysis of the Higgs-boson identification in the H → ZZ → 4l channel. The 4l final state signal and background processes are described in detail, for a range of mass from 110 to 1000 GeV. The result of the observation of a new particle in this channel with a mass near 125 GeV is presented. The following two chapters describe more specifically the work I've done. In order to estimate the significance of an excess or quantify an exclusion, a statistical analysis of the observed data is necessary. For this purpose, a model of the signal lineshape, parametric on the hypothetical Higgs boson mass, is necessary. The signal modelization is extracted from the Monte Carlo samples, which do not describe immediately the experimental data with the required precision for this analysis. Therefore, it is necessary to correct the Monte Carlo from discrepancy and possibles systematic errors. I have been work- ing on the correction of the simulated samples through the use of event weight and per-event correction factors. This is subject of chapter 4. The first part of the chapter is focused on the correction of the Pile-up distribu- tion and of the trigger, reconstruction and identification efficiencies. The second part of the chapter concerns the theoretical lineshape descripton in the high mass region. Once these corrections were applied, I could proceed with the study of the modelization of the signal shape. I worked on two different parametriza- tion, one for the low mass region and the other one for the high mass region. In the latter I have been working on the signal model systematic uncertainty estimation. This is described in chapter 5.